Pn or Schottky semiconductor junctions are at the basis of solar cells. Basically, when photons more energetic than the bandgap of the light absorbing semiconductor are absorbed by a junction, charge carriers, i.e. electrons and holes, are generated and separated by the internal electric field developed in the interfacial depletion zone. The limited charge transport through the p-n junction and the presence of energy barriers at the interfaces are factors affecting the power conversion efficiency of the current solar cells, i.e. the short-circuit photo-current density Jph and the open-circuit photovoltage-VOC).
Recently, power conversion efficiency of 25% has been reported for single-crystal silicon solar cellsi. Unfortunately such performance is still related to a number of disadvantages, including for example high material costs, high energy payback times, lack of physical flexibility and additional complicated processing steps such as providing passivation layer, anti-reflection layer (AR),ii grooves, buried contact (BC) and back surface field (BSF), etc. This is the case of the double sided buried contact silicon solar cells (DSBC) that were developed to solve the problem of hole-electron recombination from rear aluminum-alloyed region observed, due to the thermal process at high temperature and for long times, in single sides buried contact (SSBC). The SSBC cells or buried contact silicon cells have been very successful in overcoming most fundamental limitations associated with the conventional screen-printed metallization scheme.iii In DSBC cells, however, a shunt path is needed between the back surface electrode and a floating junction layer to obtain the desired BSF effect. To solve this problem, a self-biased solar cell structure is introduced to reduce the hole-electron recombination at the rear surface area.iv However, this method involves additional processing steps for connecting the front electrode to the back electrode. Furthermore, since BSF is obtained by using voltage generated by the solar cell, its dimensions are limited to values smaller than the open-circuit voltage (VOC).
Other structures based on ferroelectric films have been introduced to increase the efficiency of a single semiconductor solar cell.
Kim et al.v have suggested a method to achieve high efficiency in single semiconductor solar cells by using a ferroelectric material, by forming ferroelectric layers on the front and the rear surfaces of the semiconductor solar cell. The electric charge generated by the spontaneous polarization of these ferroelectric layers provides a surface passivation effect or a back surface field, depending on the position of the layers. Furthermore, with the deposited ferroelectric layers, the open-circuit voltage of the solar cell can be increased significantly while enhancing the energy efficiency of the single semiconductor solar cell.
Pulvarivi has proposed an efficient method based on a ferroelectric material for converting solar energy into electricity, by sandwiching, in the solar cell, a very thin film of ferroelectric insulator between a semiconductor and a metal electrode, thus forming a Metal-Insulator-Semiconductor (MIS) structure. In this case, the thermally induced electric charge produces an inversion layer used to make the desired pn junction.
In all techniques, however, loss by interfacial recombination of holes and electrons is increased due to the formation of a heterojunction at the semiconductor-ferroelectric material interface. Furthermore, the electrons mobility is limited due to the insulating effect and large band gap of the ferroelectric materials, which in turns results in lowering the efficiency of the cell.
The photovoltaic effect observed in ferroelectric perovskite thin films has recently attracted attention due to its potential applications in the area of optoelectronic devices and optical information storage. In contrast to the conventional junction-based interfacial photovoltaic effect in semiconductors (i.e. p-n or Schottky junctions), the photovoltaic effect in ferroelectrics is essentially a bulk effect: the photo-generated charge carriers of both polarities are driven by the polarization-induced internal electric field in opposite directions towards the cathode and the anode, respectively, and contribute to the photovoltaic output. Increased photovoltaic power conversion efficiency for ferroelectric thin films (around 0.28%) has been recently achieved with devices based on epitaxial La-doped lead zirconate titanate (PZT) filmsD, although the efficiency of this material remains limited by its large band gap. The availability of lower band gap multiferroic oxides such as BiFeO3 (BFO) and Bi2FeCrO6 (BFCO)E, F provides alternative materials to achieve a higher photovoltaic efficiency.
There is still a need in the art for high efficient solar cells involving simplified components structure and using simplified processing steps.